In North America platform trailers, van trailers, and intermodal containers and chassis are commonly used to transport goods. The floors of these vehicles are load bearing elements required to resist repeated applications of gravity loads from wheels of lift trucks carrying goods into and out of the vehicles. The floors typically consist of wood based decking elements and steel supporting beams.
In terms of the steel supporting beams, the basic structural system of a typical platform trailer is very different from that of typical van trailers and/or intermodal containers and chassis. Usually there are two main steel girders spaced 1.2 m apart and two smaller C-channels spaced 2.4 m apart, running along the length of a platform trailer. Perpendicular to the length of the trailer, light weight high strength steel I-beams are mounted onto the main girders forming the basic steel supporting elements. These heavy girder elements stiffen the floor and allow weaker wood based flooring panels to be used effectively to service the applied loads. Van trailers and/or intermodal containers and chassis, however, usually only consist of two small C-channels running along the length of the vehicle spaced 2.4 m apart. In effect this monocoque construction require stronger flooring material to be used effectively to service the applied loads.
The flooring panels are subjected to loading from wheels of lift trucks carrying goods into and out of the vehicles. Such loading results in shear and bending stresses in the flooring panels. Throughout the panel depth, the shear stress is highest close to the neutral axis near the centre of the panel and very small close to the edges. The bending stress, on the other hand, is zero at the neutral axis near the centre of the panel and reaches maximum value at the edge. Consequently, two common failure modes are encountered with laminated wood panels in flooring applications: shear failure near the centre of the panel or bending tension failure at the bottom face of the panel.
Hardwood species are traditionally used for vehicle decking, either a tropical species group Apitong or laminated oak. These species have superior structural and wear and tear properties compared to softwood species. In the late 80""s the continued availability of the hardwoods at a reasonable price became an issue. Furthermore, if alternate vehicle decking made from less dense softwoods were available, it would be possible to increase vehicle payloads which is an important cost incentive for the vehicle owners.
Existing softwood based panels can fulfil the industry requirements for service on platform trailers but have not been able to meet the structural requirements of the larger van trailers and intermodal containers and chassis market. The structural requirements on the flooring as stipulated by the Truck and Trailer Manufacturer Association (TTMA) consist of carrying a repeatedly applied front axle load for 3000 cycles without failure. The load rating for the van trailers and intermodal containers market spans from 73 kN to 107 kN front axle load.
U.S. Pat. No. 5,041,322 and No. 5,143,418 both to Fouquet disclose veneer flooring panels for trucks made from softwood Douglas-fir veneers pressed and bonded together by a resin to form panels of at least 11 veneers. The majority of veneers are arranged with the face grain oriented along the longitudinal direction of the panel with two cross ply veneers toward the upper and lower edges of the panel to provide stability and a small load carrying capacity in the width direction. Panels constructed according to this arrangement and tested under bending in the longitudinal direction typically fail in rolling shear in the cross plies. These results indicate that this panel does not meet the TTMA requirements.
Applicant has developed an engineered wood based laminated panel that addresses the shortcomings of the prior art. The panel of the present invention has been developed for use as structural flooring material in transportation vehicles via attachment to the steel frame of the vehicle. The panel has a unique structure that mixes high and low density veneers and resin impregnated reinforcements to create a panel of superior strength that is able to meet the structural requirements specified by the TTMA.
According to one embodiment of the invention, there is provided a wood based laminated panel with a plurality of layers in order down from a top surface, comprising: a surface layer having at least two high density wood veneer plies with the grain oriented parallel to one side of the panel; a load distribution layer having at least one high density wood veneer ply with the grain cross oriented to the one side of the panel; a core layer having from about 8 to 14 wood veneer plies with grain orientation parallel to the one side of the panel with high density wood plies, and a resin impregnated reinforcement layer.
In another embodiment there is provided a method of manufacturing the wood based laminated panel as described comprising the steps of coating the veneer plies with resin, arrange the plies in a predetermined lay-up together with the resin impregnated reinforcement layer; lacing a resin impregnated paper layer on top of the lay-up, and pressing the lay-up together at predetermined temperature and pressure to cure the resin and form the panel.